Introduction to Inheritance
Lecture 12
Fall 2008
Mendel’s Work
Gregor Mendel
• Austrian Monk
• Published work in 1866
Importance of Mendel’s work
• Determined rules of
inheritance
• Heritable factors (genes)
passed on from parents to
offspring
• Heritable factors retain their
individual identities generation
after generation
1
Mendel’s Work
Why was Mendel successful?
• Chose an appropriate study organism
– Pea plant
– Small, easy and quick to grow
– Many varieties available
• Character: heritable feature that varies among individuals
• Trait: variant of a character
– Able to control mating
– Short reproductive cycles with many offspring
• Mathematically analyzed the data he collected
– Quantitative experiments
• Luck
– Traits he studied were controlled by one gene each,
with simple dominant, recessive patterns and no
crossing over
2
3
Flower structure
Floral organs
• Sepals
– Outer whorl, often green
– Can be colored like petals
• Petals
– Attract pollinators
– Bright colors
• Stamens
– “Male” reproductive structure
– Produces pollen (sperm)
• Carpels
– “Female” reproductive structure
– Base of carpel is ovary (egg)
Fig. 9.3
Mendel’s Work
4
Controlled mating
• Pea flowers typically selffertilize
– Egg and sperm from same parent
– Pollen from stamens land on
carpel in same flower
– Prevent possibility of pollen from
other flowers by enclosing flower
in a bag
• Cross fertilization could be
controlled by hand fertilization
– Egg from one parent and sperm
from another parent
– Pollen removed by hand from
one plant and placed on carpel of
another plant
See Fig. 14.2
5
Mendel’s Work
Created true breeding lines of
individuals for each character
• Allowed plants to self fertilize
for many generations
• True breeding individuals carry
hereditary determinants for
only one form of trait
– i.e., alleles are the same
– Individuals produce only
offspring with that same
trait
See Table 14.1
Mendel’s Work
What happens when true breeding lines are
crossed?
E.g., when a purple flowered plant is crossed with
a white flowered plant
Terms
• Hybrid: offspring of two different true-breeding
varieties
• Genetic cross: cross fertilization between two
different varieties
• P generation: parental generation
• F1 generation: offspring
• F2 generation: offspring of F1 generation
6
Mendel’s Work
Terms
• Homozygous (homozygote)
– Individuals that carry identical alleles for a gene
• Heterozygous (heterozygote)
– Individuals that carry 2 different alleles for a gene
• Dominant allele
– The allele that is expressed in an organism
– Capital letter (e.g., P for purple flower)
• Recessive allele
– Allele that is only expressed if no dominant form
– Lower case letter (e.g., p for white flower)
7
Monohybrid Cross & The Law of Segregation
Monohybrid cross: cross between parent plants that differ in
only 1 characteristic
See Fig
14.3 &14.5
Punnett Square
8
Mendel’s hypotheses
9
1. There are alternative forms of genes, called
alleles
2. For each inherited characteristic, an organism
inherits 2 alleles (1 from each parent)
– Homozygous: 2 identical alleles
– Heterozygous:2 different alleles
3. In a heterozygote, one allele is dominant (is
expressed) while the other is recessive (is not
expressed)
– Dominant allele: capital letter (e.g., P for purple flower)
– Recessive allele: lower case letter (e.g., p for white
flower)
4. Law of segregation
– Two alleles for a heritable character segregate during
gamete formation and end up in different gametes
10
Alleles and Chromosomes
• Genotype
– Sequence of nucleotide bases in DNA
– All the alleles of every gene present in a given individual
• Gene : a discrete unit of hereditary information consisting of a
specific nucleotide sequence in DNA
• Phenotype
– Any observable traits in an individual
• Physical, physiological & behavioral
– The allele that is expressed
• Loci (locus)
– Specific location of genes along a chromosome
See Fig. 14.4
Dihybrid Cross and the Law of Independent
Assortment
11
What would happen in a dihybrid
cross?
– Dihybrid – mating of parental
varieties differing in two
characteristics
Two hypothesis
• The traits travel together
(dependent)
• The traits travel independently
– Seed color
• Y = yellow
• y = green
– Seed shape
• R = round
• r = wrinkled
Dominant
Recessive
Dihybrid Cross and the Law of Independent Assortment
See Fig. 14.8
12
Mendel’s Laws
Law of independent assortment:
• Each pair of alleles assorts independently of the
other pairs of alleles during gamete formation.
– The inheritance of one characteristic has no effect on
the inheritance of another characteristic
Law of segregation:
• Two alleles for a heritable character segregate
during gamete formation and end up in different
gametes
13
14
Testcross
• Dominant genes are expressed (phenotype)
• How do you tell the genotype?
– Two possibilities
• Homozygous for dominant allele (e.g., BB)
• Heterozygous with dominant allele (e.g., Bb)
• Use a test cross
– Mate individual of dominant phenotype (but unknown
genotype) with an individual of recessive phenotype
(and therefore recessive genotype)
15
Testcross
• Problems with
test cross?
Probability & the Punnett Square
Punnett Squares
– Allow for prediction of the outcome
of a particular mating
• What is the probability that two
independent events will occur
together
– E.g., two heads from two coin
tosses
– E.g., two B alleles (one from each
gamete)
• Gametes fuse randomly, so
independent events
• Each coin toss or gamete
formation is independent of the
other
See Fig. 14.9
16
Probability & the Punnett Square
• Rule of Multiplication
– For independent events, the
probability of a compound event is
the product of the separate events
• E.g. probability that an F1
generation will have BB
• ½X½=¼
• Rule of Addition
– The probability that any one of two
or more mutually exclusive events
will occur is calculated by adding
their individual probabilities
– Multiplication rule provides
individual probabilities that are
added together
• E.g., probability that an F1
generation will have a Bb
– ¼+¼=½
See Fig. 14.9
17
Variations on Mendel’s Laws
• Mendel worked with characteristics that
were controlled by simple
dominant/recessive inheritance of one
gene
• But many characteristics not that simple
– Incomplete dominance
– Multiple Alleles
– Codominance
– Pleiotrophy
– Polygenic Inheritance
18
Variations on Mendel’s Laws
19
Incomplete Dominance
• An inheritance pattern in
which the heterozygote
phenotype is a blend or
combination of both
homozygote phenotypes
Example: snapdragons
• F1: all pink
• F2: 1:2:1
Fig. 14.10
Variations on Mendel’s Laws
Multiple Alleles
• More than two different
forms of a gene
• Any individual can only
have two alleles
Example: human blood
groups
– Three alleles: A, B, O
– Six genotypes
– Four phenotypes A, B, AB, O
Codominance
• Both alleles are expressed
in heterozygous individuals
• Example: AB
– Makes both A & B
carbohydrate
Fig. 14.11
20
Variations on Mendel’s Laws
Pleiotrophy
• A pattern of genetic expression in which one
gene affects more than one phenotypic trait
Example: sickle-cell anemia and malaria
What is sickle cell anemia?
• Abnormal hemoglobin produces sickle shaped
red blood cells
– Sickled RBC’s do not carry oxygen efficiently
– Homozygous – suffer from disease
– Heterozygous – normally healthy, But
• Allele for sickle cell and normal cell codominant
• Both normal cells and sickle cells in body
• Sickle cells get destroyed over time by body defenses
21
Variations on Mendel’s Laws
Example: sickle-cell anemia and malaria
Why is sickle-cell anemia so common in Africans
and African-Americans?
– 1 in 400 African-American children have it
– 1 in 10 are heterozygous
22
Variations on Mendel’s Laws
Why is sickle cell anemia so common in Africans
and African-Americans?
• Individuals who are heterozygous for sickle-cell
anemia are more resistant to malaria
– Malaria common in many parts of Africa
– Malaria is caused by a microorganism
– Microorganism spends part of its lifecycle in a blood
cell
– In heterozygous individuals, invasion of a RBC by
microorganism causes cell to sickle
– Sickled cell, and microorganism it contains, destroyed
by body’s immune system
• Selective advantage to be heterozygous for
sickle-cell anemia
23
24
Variations on Mendel’s Laws
• Epistasis
– A gene at one locus alters
the phenotypic expression
of a gene at a second locus
– E.g., hair color in mice
– Locus for gene color
– Locus for pigment
deposition
Fig. 14.12
Variations on Mendel’s Laws
Polygenic Inheritance
• Additive effects of two or more
genes on a single phenotypic
character
• Quantitative characters
– Vary along a continuum within a
population
•
•
•
•
Example: skin color
At least three genes involved
Genes inherited separately
Genes display incomplete
dominance
– AABBCC=very dark
– aabbcc = very light
– Any other combination falls along the
gradient
25
Fig. 14.13
26
Environmental Factors
Environmental Factors
• Environmental factors can
influence phenotype, BUT
• Environmental influences
are not inherited
• Only genetic influences
(genotype) are inherited
Exception?
• Mutations from
environment (physical,
chemical) that occur in
reproductive cells
Fig. 14.14
27
Dominant & Recessive Disorders
• Dominant phenotypes
– Genotype: AA, or Aa
• Recessive phenotype
– Genotype: aa
• Dominant does not mean a phenotype is
“normal” or more common in a population
• Wild type
– Trait that is found most often in nature
– Can be recessive
28
Dominant & Recessive Disorders
• Mendel worked with characteristics that
were controlled by simple
dominant/recessive inheritance of one
gene
• Many diseases controlled by a single gene
• Most genetic disorders recessive
– Most from 2 heterozygous parents
– The closer the parents are related, the more
likely they are to carry the same recessive
alleles
• Inbreeding: mating of close relatives
29
Dominant & Recessive Disorders
• If disease is caused by a
single gene loci with a
dominant/recessive
pattern, the probability of
an offspring having that
disease can be
determined by a Punnett
square
Deafness caused by recessive
allele
Fig. 9.14
30
Dominant & Recessive Disorders
• Lethal recessive disorders more common
than lethal dominant
• Individual can be a carrier (Ll) of a lethal
recessive allele
– Allele remains in population
• If a dominant disorder is lethal:
– If it kills individual when young, then not
passed on
• allele becomes less common in population
– If it kills individual when past reproductive
age, then can be passed on
• allele remains within the population